The present invention relates to an image forming apparatus, particularly to a liquid toner development electrophotographic apparatus having a mechanism capable of performing printing on both sides of a sheet of paper by using a non-volatile, highly viscous and highly concentrated liquid toner as the developer.
As an art enabling double-sided printing in an electrophotographic recording apparatus, one based on switch back process is known (for example, refer to JP-A-5-66631 and JP-A-7-128921). FIG. 3 shows a related image forming apparatus having a switch back unit. In the drawing, a sheet of paper in a hopper is taken out by a pick up unit. The picked up sheet is transported to arrive at an image transfer section where an image is recorded on one face of the sheet. The sheet is temporarily sent to a buffer section after passing a fixing unit. After the trailing edge of the sheet passes the fixing unit and is completely accommodated in the buffer section, the sheet is switched back and transported in the opposite direction. Namely, the trailing edge for the one face recording is now transported as the leading edge in the transport direction. Thereafter, the sheet advances to a transport path beneath the fixing unit to pass the upper surface of the hopper. Then, the sheet is reversed in a sheet feed section whereby the first recording face is directed downward with the unrecorded face directed upward. Next, the sheet is sent again to the image transfer section, where image recording on the opposite face is performed and thereafter passes the fixing unit. The sheet, on both sides of which images have been recorded in this manner passes, the buffer section and is exhausted to a stacker.
Still another related art is known, in which a sheet having an image recorded on one face thereof is once stacked in a tray different from a hopper, and is transported to an image transfer section from the tray in sequence (refer to JP-A-6-27759).
In this manner, in a related electrophotographic recording apparatus, the sheet transport path for double-sided printing is such that, after a sheet has passed a fixing unit, the sheet transport path is switched to transport the sheet again to an image transfer section. But, when the sheet is transported again to the image transfer section after passing the fixing unit, the length of the sheet transport path is elongated, which increases the number of sheet wasted inside the apparatus (residual sheets in the apparatus) in case of irregular event occurrence (such as sheet jamming).
Further, with the expansion the length of the sheet transport path, the accuracy of printing position deteriorates for reverse face printing. Moreover, in case of double-sided printing, it is necessary to pass the sheet twice through the fixing unit, which doubles the consumption of the electric power for the heater.